Read here about a class of phenomena and its role in broad issues of scale. Light also has physical properties that can be measured and depicted in a scalometer.

Light Perception

Sight or vision is the sense of light perception. Photoreceptors located in the eye transduce electromagnetic radiation into electrical nerve pulses.

The human eye is capable of seeing somewhat more than a 2 trillion-fold range: The presence of white objects is somewhat discernible under starlight, at 5×10^−5 lux, while at the bright end, it is possible to read large text at 10^8 lux, or about 1,000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

See photoreceptor.

Measuring Light

The ability of light to illuminate is measured in the International System of Units (SI) in candela (cd) to represent luminous intensity, lumen (lm) or candela per steradian solid angle (cd-sr) to represent luminous flux, and lux (lx) or luminous flux per square meter (lm/m^2) to represent illuminance.

The candela is an SI base unit. It is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and has a radiant intensity in that direction of 1/683 watts per steradian.

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

In the CGS system, the unit of illuminance is the phot, which is equal to 10,000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.


Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.